As conventional electrolytic solutions for electrolytic capacitors, there are known such electrolytic solutions as one using a tertiary ammonium salt of an aromatic carboxylic acid such as phthalic acid and the like as an electrolyte salt, one using a tertiary ammonium salt of maleic acid as an electrolyte salt, one using an electrolyte salt containing amidines as a cationic component, and the like.
This electrolyte salt containing amidines as the cationic component has a high specific conductivity and can make the resistance of the capacitor lower. Also, because electrolysis products disappear rapidly due to a reaction of hydroxide ions formed by an electrochemical reaction and amidine groups, N—C—N, this electrolyte salt does not deteriorate or corrode the resins, rubbers, and metals which are materials constituting the capacitor and thus has a characteristic of excellent reliability.
However, despite its excellent characteristics, this electrolytic solution using an electrolyte salt comprising amidines as the cationic component, has had a problem that the withstand voltage thereof is low, the withstand voltage being an indispensable characteristic for the electrolytic capacitor, especially for the aluminum electrolytic capacitor.
In order to solve the above-described problem, there has been proposed an electrolytic solution for driving the electrolytic capacitor, obtained by adding at least one selected from a polyhydric alcohol, boric acid, and a complex compound formed between boric acid and a polyhydric alcohol to an electrolytic solution which comprises amidine salts and wherein the concentration of the electrolyte salt is 5-25 wt % (see, for example, Patent Document 1).
On the other hand, recently, the switch from leaded solder to lead-free solder is progressing for the purpose of decreasing the amount of use of environmental loading materials. In order to respond to this lead-free solder, it is necessary to raise the temperature of the reflow process to 260° C. However, according to the method of the above-described Patent Document, condensation water is generated from the polyhydric alcohol, boric acid, and the complex compound formed between boric acid and the polyhydric alcohol, resulting in high water percentage inside the capacitor. The water inside the capacitor evaporates by heat (for example 260° C.) inside the oven for reflow of lead-free solder (hereinafter abbreviated as reflow) and the internal pressure rises, causing occurrence of expansion of the capacitors and thereby making it difficult to surface mount the capacitors on the substrates. In other words, it is necessary to maintain the water content in the electrolytic solution low in order to suppress the expansion caused by reflow.
Therefore, there has been proposed an electrolytic solution in which the polyhydric alcohol, boric acid, and the complex compound formed between boric acid and polyhydric alcohol, which generate water, are not used but which comprises an alkyl phosphate anion having a high sparking voltage (see, for example, Patent Document 2).